The present invention relates to an information recording medium recording method, information recording/playback apparatus, and information recording medium and, more particularly, to a recording method of stabilizing continuous recording on a high-density information recording medium, an information recording/playback apparatus using the method, and an information recording medium on which recording is performed by the recording method and recording/playback apparatus.
With the diversification of information, data handled in the storage field have also increased in amount. In the field of optical disks as well, efforts have been made to achieve an increase in capacity by increasing densities from those of CDs to those of DVDs. As attempts to develop techniques toward higher density, there have been developed a technique of accurately recording marks as small as possible and a technique of allowing playback even near an optical playback limit. Conventional recordable DVDs will be described first.
Conventionally, as recordable DVDs, optical disks such as DVD−RAMs, DVD−Rs, DVD−RWs, DVD+Rs, and DVD+RWs have already been on the market. Optical disk apparatuses which record and play back on these disks include an apparatus having a recording speed as high as octuple speed. Note, however, that such an apparatus cannot record on the entire disk layer at the highest speed. Instead, the disk layer is divided into several areas in the direction from the inner circumference to the outer circumference, and the apparatus changes the linear speed in the respective areas so as to execute recording at speeds as high as possible. Such an optical disk has an area (PCA: Power Calibration Area) for adjusting recording power on a portion of the disk area. The apparatus is designed to properly perform recording power adjustment (OPC: Optimum Power Control) by using this area.
The optical disk apparatus records actual data by using the recording power obtained at this point of time. As recording power adjustment methods, there are known the β method of obtaining a β value by checking asymmetry on the basis of the playback amplitude of a long mark and the playback amplitude of a short mark and the γ method of determining a state from the degree of saturation of a recording mark amplitude. The written standards concerning 2× (double speed) of a DVD−RW disk describe the OPC flag indicating the necessity of a change in asymmetry value by 5% or more with a change of 15% in recording power as the accuracy of the change amount of β value when the β method is used.
In actual recording, recording performance is maintained by using a technique called R-OPC (Running-OPC). According to this technique, at the time of OPC in a PCA, the amount of light returned from the disk during recording is measured, the amount of return light at the time of the optimal β value is set as a target value, and the recording power is changed to make the amount of return light during actual data recording become equal to the target value.
Playback techniques will be described next. Conventionally, a slice identification scheme has been used for the binarization of data. This technique has used a technique of filtering a playback waveform by using an equalizer so as to reduce intersymbol interference. In this case, the equalizer suppresses intersymbol interference but increases noise components. This makes it difficult to decode recorded original data from a playback signal at the time of high density.
In contrast, as a technique of accurately decoding data recorded at a high density, the PRML (Partial-Response Maximum-Likelihood) technique is effective. In this technique, partial response (to be abbreviated as PR hereinafter in some cases) equalization (PR equalization) of a playback waveform is performed to obtain a waveform having intersymbol interference so as not to increase noise components, and data identification is executed by a technique called Viterbi decoding (ML). PR equalization is defined by an amplitude for every data period (clock). For example, PR(abc) indicates that the amplitude at time 0 is represented by a, the amplitude at time T is represented by b, the amplitude at time 2T is represented by c, and the amplitude at other times is 0. The total number of components with amplitudes other than 0 is called a constraint length. In order to increase density, it is effective to use a partial response waveform with a long constraint length. In contrast, using a waveform with a long constraint length is premised on the use of a waveform with large intersymbol interference.
The PR(1, 2, 2, 2, 1) characteristic will be described as an example. The PR(1, 2, 2, 2, 1) characteristic indicates a characteristic in which a playback signal with respect to code bit “1” is represented by “12221”. Convolution computation of a code bit sequence and sequence 12221 representing a PR characteristic generates a playback signal.
For example, a playback signal with respect to code bit sequence 0100000000 is represented by 0122210000. Likewise, a playback signal with respect to code bit sequence 0110000000 is represented by 0134431000; a playback signal with respect to 0111000000, 0135653100; a playback signal with respect to 0111100000; 00135775310; and playback signal with respect to 0111110000, 0135787531. According to the PR(1, 2, 2, 2, 1) characteristic, playback signals have nine levels. The playback signal calculated by such convolution computation is an ideal playback signal (path).
A playback signal, however, does not necessarily have the PR(1, 2, 2, 2, 1) characteristic, and contains deterioration factors such as noise. In PRML detection, therefore, a playback signal is approximated to the PR characteristic by an equalizer (a playback signal approximated to the PR characteristic will be referred to as an equalized playback signal). Thereafter, a Viterbi decoder is used to select a path with the shortest Euclidean distance from the equalized playback signal. This path has a one-to-one relationship with a code bit sequence. The Viterbi decoder which performs Viterbi decoding operation outputs a code bit sequence corresponding to the selected path as decoded binary data. A system using PRML is based on the premise that a playback signal is a ternary or higher-order signal, so-called a multilevel signal, instead of a binary signal. In slice identification detection, the presence/absence of a pit is determined on a proper slice, and data is played back upon binary equalization. Therefore, in PRML detection premised on multilevel signals, unlike in the former case, recording/playback waveforms must conform to PRML detection.
FIG. 29 shows examples of measurement of error rates in cases wherein conventional binary equalization with slice identification is used and conventional PRML detection is used while the pit length is changed. The broken line represents the error rates in binary equalization; the chain line, an allowable value reference for an apparatus; λ, the laser wavelength of a light source; and NA, the numerical aperture of an objective lens. It is obvious from FIG. 29 that conventional PRML detection can perform playback with smaller pits, and the shortest pit length in slice identification is limited at about 0.35×λ/NA. Note that the pit lengths in conventional DVDs are about 0.37×λ/NA.
In Japanese Patent Laid-Open No. 2002-197660, the inventor of the present application discloses a detection means corresponding to an asymmetry in a case wherein PRML detection is used. In this case, an asymmetry detection circuit includes a timing adjustment circuit which receives a digitized sample value, a Viterbi detector which receives the sample value, a reference level determining device which receives the Viterbi detector output, a filter circuit which receives the Viterbi detector output, an error calculator which calculates the difference between the filter circuit output and the timing adjustment circuit output, a plurality of discrimination circuits each of which receives the reference level determination circuit output as a discrimination signal and discriminates the error calculator output, a plurality of integration circuits which integrate the plurality of discrimination circuit outputs, an average calculation circuit which calculates the average of the maximum reference level integrated value and the minimum reference level integrated value of the integration circuit outputs, and a circuit which executes the operation of calculating the difference between the average value and the median reference level integrated value of a plurality of reference level integrated values.
Even if the recording power of laser light is optimal at the start of data recording, the power may deviate from the optimal state during recording due to various factors caused by the characteristics of the disk medium itself and the use state of the optical disk apparatus. That is, different situations may occur within a single disk depending on the place (area) and the state of the apparatus. In this case, the recording quality deteriorates in such a portion. In addition, correction by conventional R-OPC cannot cope with such a problem.
In order to keep recording quality constant, for example, Japanese Patent Laid-Open No. 2003-331426 discloses an optical disk apparatus which stops recording and obtains recording power again during data recording on an optical disk, and resumes recording with the obtained recording power of laser light, an optical disk apparatus which stops recording during execution of recording on an optical disk, detects the recorded state of the data, and obtains the recording power of the laser light again on the basis of the detection result, and an optical disk apparatus in which power is determined on the basis of the relationship between a recording state and recording power at the time of OPC, and if the recording state exceeds an allowable range, power is obtained again, and if the recording state falls within the allowable range, recording is resumed without any change in power. According to this reference, the timing of calculating recording power again and the number of times recording power is calculated are not limited, and are properly set as needed. That is, such calculation is performed randomly at arbitrary timings, e.g., at predetermined time intervals or every time a predetermined amount of data is recorded.
In a case wherein this technique is used, a β value is exemplified as a state detection value derived from a playback signal (HF signal) read out from an optical disk.
According to Japanese Patent Laid-Open No. 2003-281720, when the recording power corrected by R-OPC reaches the maximum value, recording operation is interrupted, and resumed after a lapse of a predetermined period of time. In addition, when the interruption exceeds a predetermined period of time, an error is notified to a host apparatus.
The relationship between an information recording/playback apparatus which actually performs recording/playback and a host system which transmits a command to record information and information to be recorded will be described, especially in a case wherein the apparatus is operated by using the drive control command described in ANSI.T10/1545-D or SFF8090i. A convention by which a constant recording rate is expected with the MMC command will be described. In general, a host application uses the command “Getperformance” to check the maximum speed at which recording can be done from the apparatus to a disk. The host application then uses SetStreaming command to determine how many Kbytes of data should be supplied per unit time. At this time, Streaming bit assigned to the seventh bit of the 10th byte of WRITE (12) command is set to 1. When this Streaming bit is effective (“1”), the host application operates on the assumption that the apparatus guarantees that the transfer rate is kept constant.
First of all, the results confirmed by the inventor of the present application and the like will be presented. The following is the experimental result in which conventional β values were obtained when the power was changed at a shortest bit length of 0.13 μm/bit in (1, 7) RLL with respect to a phase-change disk having a substrate thickness of 0.6 mm with an NA of 0.65 (of the objective lens of an optical head) and an LD wavelength λ of 405 nm. Referring to FIG. 17, each black circle (solid line) represents a bER (bit error rate), each black triangle (chain line) represents β obtained by the conventional technique, and each black rectangle (broken line) represents β detected by a detection arrangement suitable for high density, which is expressed by β2. In this arrangement, data was not able to be accurately played back without using the PRML detection technique. As shown in FIG. 17, obviously, when data is recorded at a density high enough to use PRML detection, the detection accuracy in the conventional detection method lacks 5% or less with respect to a change of 15% in recording power. It is impossible to keep the recording quality constant using this, thus posing a problem in high-density recording/playback operation.
The β value of a playback signal is not an index indicating the absolute value of performance. FIG. 18 shows an example of actual measurement on the correspondence between β values with respect to powers and bit error rates. Even with the same β value, the minimum values of bit error rates do not coincide with each other, and the margin curves represented by error rates are not in a one-to-one correspondence with β value deviations in some cases. Obviously, therefore, a β value is an index which well represents a relative amount indicating a power deviation but is not good at representing the absolute value of performance. That is, a β value deviation may not indicate how much the absolute value of performance deteriorates.
In addition, there is no ground for a criterion for determination as to the timing of correcting the recording power. Since there is no inevitability about the execution of correction at predetermined time intervals or for each predetermined data amount, unnecessary operation must be executed.
Furthermore, consider an optical disk apparatus which interrupts and resumes operation. It is a problem for a host system which exchanges information with such an optical disk apparatus that the state of the apparatus is unknown, because the apparatus does not operate independently. Assume that the host system operates the apparatus by using the drive control command described in ANSI.T10/1545-D or SFF8090i (Mt. Fuji). In this case, when the apparatus interrupts upon executing interruption operation or the like, the host system cannot record data at a predetermined rate in spite of the fact that the host system expects to perform recording at the predetermined rate. In such a case, i.e., a case wherein a response indicating the reason for interruption is not returned, since the overall system does not stop, the host system generally performs operation such as stopping the apparatus due to a timeout error or the like or performing an alternative procedure upon autonomously determining another reason. As a result, unexpected operation may occur, and desired operation cannot be done, leading to the failure of the apparatus.